In accordance with prior art, a DC power supply, e.g. AC/DC switching power supply, usually employs a high frequency pulse width modulation (PWM) control circuit to control the output voltage for reducing the entire volume. As shown in FIG. 1, which is a schematic diagram of a conventional AC/DC flyback switching power supply. The transformer T1 of the conventional AC/DC flyback switching power supply has two sides that are designated as primary side 101 and secondary side 102, respectively. The electrical signals of the primary side 101 and secondary side 102 are parted by a photodiode 111 and a light emitting diode (LED) 112. But, the variation of the voltage or current of the secondary side 102 can be fed back to the primary side 101 by an optical signal to synchronously adjust the voltage and current of the primary side 101 and secondary side 102. The optical signal can also be used as an over-current or short-circuit protection feedback signal.
The conventional AC/DC flyback switching power supply is a circuit, which is mainly used to transform an alternating voltage to a direct output voltage. Please refer to FIG. 2, which shows output waveforms of several components. The operation principle of the conventional AC/DC flyback switching power supply is as following: while the FET Q1 of the primary side 101 is turned on, since the diode D1 will be biased by a reverse voltage, the electrical energy will be stored in the transformer T1; while the FET Q1 is turned off, the diode D1 will be biased by a forward voltage and the stored energy will be released from the transformer T1. However, when the diode D1 is biased forwardly, the diode D1 will cause a voltage drop ranging from 0.4 V to 1.5 V, depending on the value of the output load. The value of the voltage drop multiplied by the value of the output current is the value of the power loss. If the output current is huge, the efficiency of the conventional AC/DC flyback switching power supply will drop substantially.
Accordingly, as discussed above, the conventional AC/DC flyback switching power supply still has some drawbacks that could be improved. The present invention aims to resolve the drawbacks in the prior art.